Cellular respiration requires many oxidation-reduction reactions which involve half-reactions, electron carriers, and flavoproteins.
Cellular respiration involves many reactions in which electrons are passed from one molecule to another. Reactions involving electron transfers are known as oxidation-reduction reactions (or redox reactions), and they play a central role in the metabolism of a cell. In a redox reaction, one of the reacting molecules loses electrons and is said to be oxidized, while another reacting molecule gains electrons (the ones lost by the first molecule) and is said to be reduced. You can remember what oxidation and reduction mean with the handy mnemonic “LEO goes GER”: Lose Electrons, Oxidized; Gain Electrons, Reduced.
Every balanced redox reaction is composed of two half-reactions: the oxidation half-reaction, and the reduction half-reaction. See below for an example of a reduction half-reaction and an oxidation half-reaction with its net reaction:
In the reaction example it can be seen that copper (Cu2+) gains electrons in this case and magnesium loses electrons becoming a positive magnesium ion (Mg2+).
In biology this plays a part in the cell via electron carriers which are small organic molecules that readily cycle between oxidized and reduced forms and are used to transport electrons during metabolic reactions. There are two soluble electron carriers that play particularly important roles during cellular respiration: NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide). Both NAD+ and FAD can serve as oxidizing agents, accepting a pair of electrons, along with one or more protons, to switch to their reduced forms. NAD+ accepts two electrons and one H+ to become NADH, while FAD accepts two electrons and two H+ to become FADH2. NAD+ is the primary electron carrier used during cellular respiration, with FAD participating in just one (or two sometimes two) reactions.
These electron carriers often form together to form electron transport chains. A good example of this is found in the membrane of the mitochondria. Electrons through a series of electron transporters where NADH and FADH2 undergo redox reactions this causes hydrogen ions to accumulate within the matrix space. Therefore, a concentration gradient forms in which hydrogen ions diffuse out of the matrix space by passing through ATP synthase. The current of hydrogen ions powers the catalytic action of ATP synthase, which phosphorylates ADP, producing ATP.
Flavoproteins are proteins that have a FAD or FMN (flavin mononucleotide) and are one of four major classes of electron carriers involved in both eukaryotic and prokaryotic electron transport systems during cellular respiration. An electron transfer flavoprotein (ETF) or electron transfer flavoprotein complex (CETF) is a flavoprotein located on the matrix face of the inner mitochondrial membrane and functions as a specific electron acceptor for primary dehydrogenases, transferring the electrons to terminal respiratory systems such as electron-transferring-flavoprotein dehydrogenase.
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Key Points
• Reducing agents get oxidized, and therefore lose electrons.
• Oxidizing agents get reduced, and therefore gain electrons.
• Remember the mnemonic device OIL RIG—” Oxidation Involves Loss” and “Reduction Involves Gain” to distinguish between oxidizing and reducing agents.
• Half reactions can be used to show an element and its loss or gain of electrons
• An electron transport system (ETS) is composed of a series of membrane-associated protein complexes and associated mobile accessory electron carriers. Electron carriers can readily oxidize and reduce themselves to move elevtrons
• There are two soluble electron carriers that play particularly important roles during cellular respiration: NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide).
• Flavoproteins are proteins that have a FAD or FMN (flavin mononucleotide) and are one of four major classes of electron carriers involved in both eukaryotic and prokaryotic electron transport systems during cellular respiration.
• An electron transfer flavoprotein (ETF) or electron transfer flavoprotein complex (CETF) is a flavoprotein located on the matrix face of the inner mitochondrial membrane and functions as a specific electron acceptor for primary dehydrogenases, transferring the electrons to terminal respiratory systems such as electron-transferring-flavoprotein dehydrogenase.
Key Terms
oxidation: a reaction in which an element’s atoms lose electrons and its oxidation state increases
reduction: a reaction in which electrons are gained and oxidation state is reduced, often by the removal of oxygen or the addition of hydrogen
half-reactions: one of the two constituent parts of any redox reaction in which only oxidation or reduction is shown
cellular respiration: a set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products
soluble electron carriers: an electron carrier is a molecule that transports electrons during cellular respiration
flavoproteins: enzymes that require FMN or FAD as cofactors
electron transport system: a series of electron transporters embedded in the inner mitochondrial membrane that shuttles electrons from NADH and FADH2 to molecular oxygen
redox reactions: chemical reactions where one element is oxidised and the other is reduced
NAD+ (nicotinamide adenine dinucleotide): a soluble electron carrier that can act as an oxidizing agent, accepting a pair of electrons
FAD (flavin adenine dinucleotide): a soluble electron carrier that can act as an oxidizing agent, accepting a pair of electrons
dehydrogenase: an enzyme that oxidizes a substrate by reducing an electron acceptor, usually NAD+